We studied high-grade metamorphic rocks of the El Cristo hill area of the Tandilia belt. Mineral analyses and thermodynamic calculations were carried out for two adjacent rock samples: an amphibole–biotite gneiss and a garnet–biotite-bearing migmatite. Peritectic garnets in the migmatite show core compositions of pyr4.5(gro + andr)10spes6alm79.5 changing to pyr3.5(gro + andr)17spes6alm73.5 at their thin rims. Garnet compositions in the gneiss are pyr6.5(gro + andr)26spes12alm55.5 and pyr4.5(gro + andr)34spes12alm49.5 for core and rim, respectively. A P–T path was constructed by calculating pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H and contouring them by isopleths for garnet components using the PERPLE_X software package. Supra-solidus crystallization of garnet cores in the migmatite began at 5.8 kbar and 660 °C. Garnet rims equilibrated at 7.0 kbar and 640 °C compatible with garnet cores in the amphibole–biotite gneiss (7.6 kbar and 660 °C). The further chemical development of garnet in this rock points to P–T conditions of 11.6 kbar and 620 °C and 12.2 kbar and 595 °C (outermost garnet rim). At this high-pressure stage Ca-amphibole was not stable. Most biotite formed during exhumation whereas the high-pressure accessory minerals, titanite and epidote, persisted. According to the obtained anti-clockwise P–T path the originally partly melted material was tectonically transported from ∼22 km (middle crust) to ∼40 km (lower crust) depths reaching a geothermal gradient as low as 15 °C km−1. This transport probably occurred along a major suture zone, which was active during the Paleoproterozoic (2.25–2.10 Ga), before a terminating collision of terranes near the SW boundary of the Rio de la Plata craton. 相似文献
AbstractAbstract Accurate estimates of water losses from mature Sitka spruce (Picea sitchensis) plantations in the UK uplands are required to assess the sustainability of water supply in the event of land-use change. Many investigations have demonstrated that afforestation increases water losses from temperate upland catchments, to up to 40% of annual site rainfall. In a 0.86 km2 upland water supply catchment in southwest Scotland, interception loss in a Sitka spruce-dominated 37-year old plantation, was 52% of annual precipitation (2912 mm), considerably higher than reported in previous studies of similar catchments. From direct measurements of rainfall, cloudwater, discharge and soil evaporation, the catchment water balance was 96–117% complete, within the limits of measurement error. The most probable explanation for the higher forest interception loss reported here is the inclusion of cloudwater measurements. 相似文献
A proposal is set out for the information and details that should accompany the publication of geochemical data in the research literature. This proposal is based on the principle that sufficient detail must be included both to allow independent replication of the results and for reviewers to confirm that data are fit‐for‐purpose in supporting the way in which they have been interpreted. In particular, it is recommended that all analytical measurements should be accompanied by an estimate of uncertainty that includes both field sampling and laboratory contributions together with a statement that summarises the way published data conform to the principles of traceability. 相似文献
Magnesium isotopic compositions are reported for twenty‐four international geological reference materials including igneous, metamorphic and sedimentary rocks, as well as phlogopite and serpentine minerals. The long‐term reproducibility of Mg isotopic determination, based on 4‐year analyses of olivine and seawater samples, was ≤ 0.07‰ (2s) for δ26Mg and ≤ 0.05‰ (2s) for δ25Mg. Accuracy was tested by analysis of synthetic reference materials down to the quoted long‐term reproducibility. This comprehensive dataset, plus seawater data produced in the same laboratory, serves as a reference for quality assurance and inter‐laboratory comparison of high‐precision Mg isotopic data. 相似文献
A comparison is made between a set of current measurements in a section across the Strait of Juan de Fuca and those recorded further upstream at Race Rocks at a single depth. No outstanding common qualitative features are noted. Cross spectra reveal a low correlation for the subtidal current and suggest an intensification of the diurnal modulation of the flow at Race Rocks. The high correlation in the tidal bands allows to calculate this portion of the tidal current at Race Rocks which is coherent with the one sensed across the section. 相似文献
Exploration within the Slave craton has revealed clusters of kimberlite intrusions, commonly with internally consistent geochemical and temporal characteristics. Translation diagrams (“Fry analysis”) allow an unbiased geometrical examination of the distance and direction between each kimberlite occurrence and all others in the database. Recurrent patterns are visually accentuated due to the square function in data density. Circular histograms quantify the azimuthal density of kimberlite at various distances. For this study, the database comprises the geographic position of 212 kimberlite occurrences of which 70% are from the Lac de Gras field (LDG). Analyses are presented separately for the LDG data and for all non-LDG data in order to test for regional variations and to avoid overwhelming the craton-scale studies by the high density of LDG data.
Empirical grouping of kimberlite locations results in delineation of five elliptical clusters that encompass all but four kimberlite occurrences. Clusters within the western part of the craton are elongate to the north–northeast and align within a narrow zone (“Western Corridor”). Elsewhere, the clusters are elongate to the northwest or west–northwest and appear to be arranged en echelon within a poorly defined north–northwest trending zone (“Central Corridor”). Geometrical spatial analyses of kimberlite locations highlight the craton-scale pattern of emplacement within the two main corridors. At regional and local scales, individual intrusions are preferentially located towards the west–northwest (ca. 280°) and north–northeast (ca. 015°) of other intrusions, and these orientations are interpreted to reflect upper mantle trends in magma generation. At local scales (10–25 km), kimberlite of the central and southern craton tends to be located to the northeast (ca. 045°), and possibly weakly to the east–northeast (ca. 070°), of other intrusions, and these orientations correspond to major crustal fractures systems. It is proposed that kimberlite emplacement is controlled primarily by the interaction of elongate 280° and 015° source regions with near-surface deviations influenced by crustal fracture systems.
The 015° trend evident at craton, regional, and local scales is parallel to a swarm of alkaline diabase dykes that are concentrated in a ca. 30-km-wide corridor passing through Lac de Gras. A profound spatial association between significantly diamondiferous kimberlite and the margins of the dyke corridor suggests the corridor is the surface expression of a mantle-depth structure. It remains unclear whether the proposed mantle structure coincides with a diamond-rich zone near the base of the lithosphere, or delineates pathways favorable for diamond preservation during emplacement. The linear array of kimberlite within the western craton forms a parallel corridor that may be an analogous mantle structure, but which to date has failed to yield economic diamond concentrations. 相似文献
Gas hydrates are solid-like substances naturally occurring beneath the oceans and in polar regions. They contain vast, and potentially unstable, reserves of methane and other natural gases. Many believe that, if released in the environment, the methane from hydrates would be a considerable hazard to marine ecosystems, coastal populations and infrastructures, or worse, that it would dangerously contribute to global warming. On the other hand, hydrates may contain enough natural gas to provide an energy supply assurance for the 21st century. This paper attempts to separate the myths, the facts and the issues that relate to natural gas hydrates beyond the doomsday environmental scenarios and overly optimistic estimates. To cite this article: B. Beauchamp, C. R. Geoscience 336 (2004).相似文献
The Ría de Ribadeo, a small embayment in NW Spain, was observed for the first time using three months of ADCP (Acoustic Doppler Current Profiler) current meter records and three CTD (Conductivity Temperature Depth) surveys. A wave driven circulation pattern arises from the observations, instead of the shelf wind driven circulation of other Galician Rias. Based on ADCP records, a unidirectional flow in the horizontal velocity field coexists with the classic two-layer circulation (bidirectional flow). The proposed horizontal circulation pattern can then be explained as a wave driven inflow. This flow enters the ria through the west side, describes an anticlockwise gyre inside of it, and flows out through the east side. Numerical simulations run with the HAMSOM (Hamburg Shelf Ocean Model) agreed with observations, confirming the proposed pattern. 相似文献